The Effect of Crustal Structure on Station Magnitude Anomalies (Magnitude Bias).
TELEDYNE GEOTECH ALEXANDRIA VA SEISMIC DATA ANALYSIS CENTER
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The amplification of P wave amplitudes by the receiver crustal structure was modelled for 34 LRSM stations by using crustal structures derived from geological and geophysical information in the literature. Crustal amplification was found to be linearly related to the acoustical impedance in near surface materials at each site. Correlation of the crustal amplification, expressed in magnitude units, with the magnitude residuals of Booth, Marshall and Young 1974 is statistically not significant for the complete set of stations investigated by the above authors. This indicates that crustal amplification alone cannot explain the observed magnitude residuals. If two subsets of stations in Western and Eastern United States, respectively, are taken, a linear trend between the computed crustal amplification and the magnitude residuals can be seen within each set, but there is a separation of about 0.3 magnitude units between the two groups. The most likely explanation of the separation is anelastic attenuation in the mantle under the Western United States. About 75 of variance in the magnitude residuals, for the US stations used, can be correlated with anelasticity and crustal amplification as known at present. The residual variance must be due to uncertainties in the above factors and other causes.